Phase comparator for horizontal sweep deflection circuit



ug. 14, 1951 K. R. WENDT PHASE coMPARAToR, FOR HORIZONTAL swEEPDEFLECIION CIRCUIT Filed oct. 2:5, 194s Y AAAAAA AAA AA AAAAA PatentedAug. 14, 1951 PHASE CONIPARATOR FOR HORIZONTAL SWEEP DEFLECTION CIRCUITKarl R. .Wendt,

Eggertsville, N. Y., assignor to Radio Corporation of America, acorporation of Delaware Application October 23, 1948, Serial No. 56,245

46 Claims.

This invention relates to cathode ray beam deection systems fortelevision and the like, and more particularly to automatic frequencycontrol for deflection signal generators.

It is well known in the television art that the transmission of visualinformation by electricity is characterized by the fact that a singleelectrical transmission circuit can carry but one information item at atime. Generally these items of information are conveyed by currentpulses that are caused to ow through the transmission medium or circuit.

By analyzing the light detail of an image into its image elements andderiving therefrom a signal train of impulses by an orderly sequence ofscanning, the image may be reproduced at a remote location byreconstruction of the image in the same orderly sequence of scanning.Since the scanning and the image repetition pulses are essentiallyartificial ones, the total number of scanning lines and the total numberof image elements in each line, the sequence of transmission of thelines, the aspect ratio of the image, and the rate of image repetitionmay be chosen arbitrarily.

It is important, however, that the scanning at the receiving stationcorrespond precisely to the scanning at the transmitting station.

Synchronism in scanning at the various locations is established by thetransmission of synchronizing pulses generally included with the imagedetail information signal.

The operation of various synchronizing systems are quite well shown anddescribed in the published art. For example, the early articleTelevision Synchronization by E. W. Engstrom and R. S. Holmes inElectronics for November 1938 relates generally to the subject matter ofsynchronization.

Cathode ray beam deflection in the receiving station is controlled by asawtooth wave generator. Normally the sawtooth wave generator is drivenby an oscillator whosefrequency of oscillation is controlled by thereceived synchronizing pulses. Circuit arrangements involving sawtoothwave generators excited by oscillators whose -frequency is controlled byincoming signal pulses are also well known to the television art and areincluded in receivers-which are presently on the market. A suitabledescription of some receivers involving such arrangements may be foundin an article entitled Television Receivers published inthe RCA Reviewfor March 1947.

The subject of automatic frequency control of the oscillator frequencyused in controlling the deflection signal generator is also quite wellknown and is referred to in an article entitled Automatic Frequency andPhase Control of Synchronization in Television Receivers by K. R. Wendtand G. L. Fredendall in the Proceedings of the Institute of RadioEngineers for January 1943.

In general, automatic frequency control systems operate to compare thelocally generated wave with the incoming signal train of synchronizingimpulses. Upon comparing the locally generated wave with the incomingsynchronizing impulses a control potential may be developed which isemployed to control the frequency of the local oscillator by controllinga reactance tube, by controlling grid bias of the oscillator or anyother frequency control arrangement.

Heretofore various automatic frequency control arrangements have beenproposed in television for developing by local oscillation a wave whichis to be compared with the incoming synchronizing impulses. In thisregard it must be emphasized that the lack of delay in automaticfrequency control circuits is important since the automatic frequencycontrol circuit consists of a feedback loop which will oscillate ifinsuincient damping is used. Even in such a circuit where oscillationwill not occur because of the damping employed, 'the circuit will tendto swing about a mean value for a few cycles when it has been disturbed.The best means of avoiding oscillations of this sort is to reduce alltime delays to a minimum.

According to this invention a circuit arrangement is proposed whereindelays are reduced to a minimum and thus stability is improved.

Damping adjustments are also made very uncritical.

According to this invention a voltage is developed from the platecurrent of the horizontal output tube. This developed voltage is thenseverely differentiated to form a sawtooth wave which is then comparedwith the incoming synchronizing pulses.

In addition to the advantage gained by the reduction of delays, thesevere differentiation prevents the passage of low frequency componentswhich normally cause instability in the normal horizontal deflection.Furthermore, the impedance of the automatic frequency control circuit isso low that the automatic frequency detector cannot kick back a voltageto the source of the sawtooth voltage.

A primary object of this invention is to provide an improved televisiondeflection system.

Another object of the invention is to provide for improved automaticfrequency control of television deflection systems.

Still another object of this invention is to eliminate irregularities indeflection resulting from delay and feedback in the automatic frequencycontrol circuit.

Other and incidental objects of the invention will be apparent to thoseskilled in the art from a reading of the following specication and aninspection of the accompanying drawing wherein the drawing shows bycircuit diagram -`one form of this invention, wherein the heavy lines ofthe drawing refer more specifically to the details of the invention.

Turning in detail to the drawing there is indivcated at the upper lefthani "corner means for capacitively coupling a received televisionsignal including both an image signal component and Asynchronizingcomponenttothe input of a typical television 'receiver lbietectorcircuit. Such a composite 'signal'may of lcourse, be obtained byemploying any 'of the Vwell known television receiver circuitsfsuchas,for example, like those vshown and described 'in the article TelevisionReceivers referred to above.

The input signal including both the image signal and synchr'niz'ing'pulses 'are applied to a detector 'l,'whi ch, for Athe purpose 4ofillustration, is Yindicated"as are'ctie'r. direct current coupling isvprovided t its associated 'video amplifier Detector circuitsa'swellasvideojampliflers are lalso well known to the art and it isnotintended here to 'stutifn detail circuit components or theory of peratin"of "detector circuits or video amplifiers. It is believed:Sufcientfl'iere to note that the composite 'sign'V 'l including theimage `and lsynchrCfnifzi'nf'g components a're applied to tubes 1 and `9which'areincluded 'a circuit arrangement cornmoi'ly 'known asasynchronizing signal'separatr.

vAnfy suitable synchronizing 'signal separator .may be employed 'in 'thepractice of this invention.

Such as, Afor cir-ample, any 'of those shown and described in the'article Television Receivers Vreferred to above or asynchroni'zingsignal separatorsuch'as `shwn andfdescribedfin the U. S. VPaterna to -A.V. Bedford N`of2,`20'1,775, dated Juli7 The synchronizing "signaluseparator illustrated inthe drawing and involving tubes 1 vand 3 is,however, 'a preferred form which is vvshown and described indetailin'in'y 'c'pending U. S. application entitlvedSynchrrizing'si'gnal Separator, Serial No.'56;2'46,"led`0ctober 23,1948.

In accordance with the operation of synchronizing signal `separatcrs'the`video zportion of the composite signal is fe'd'to control electrode Ilyof kinescope 1 3 while kthe Vvertical synchronizing pulses yareseparated Vfrom the horizontal pulses incircuit 15in accordance withwell known procedure to beapplied to the vertical synchronizing signal'oscillator'nol shown. In the explanation of the opration f inventionreference will be directedprimarily to the horizontal deflection portionof the scanning.

The horizontal oscillator selected for the purpose of the explanationofthe operation'of this invention is of the multivibrator type involvingtubes I1 a'rid I9. Although anysuitable oscillator circuit may beemployedthe particular form involving tubes l1 and I9 and 'shown indetail in the drawingV isv arranged to 'scillate' at' a `frequentraceperiods of ythe cathode fray sca cy dependent not only upon circuitconstants but upon the bias potential applied to the controlY electrode2l of tube l1.

Basically the multivibrator circuit involving tubes l1 and I5 is atwo-stage resistance-coupled amplifier in which the output of the secondtube i9 vsupplies the input "of the -rst tube l1 with feedback energy tocause oscillation. Ari inductance 23 and condenser 25 are provided inthe anode circuit of tube I1. A more detailed explanation of the'operation of multivibrator circuits may be found in any of the radiotextbooks, such las for example, the Radio Engineering Handbook byu. E.frermenrirst Edition.

The -outr'i'ut 'signal of the oscillator involving tubes l1 and VI9 isapplied to the control electrode of 'the deflection power output tube21.

The .power output tube 21 produces in a transformer Z9 a current whichin combination with the dualdiode tube 31, known as the damper tube,Aproduces a constant voltage l'across horizontel denectioneolls y33top'r'odce in winnie-linear `deilection required for kinescope f3.

Generally the 4deflection circuits ofthe type shown v'and employing the`so calle'd damper tube 3l, causel a current to -ilow through fthedamper tube 3l Yduring scansion as -a result `o'l"theenergy stored inthelcircuit inductance during the -rening cycle. This current theoreticallymay be Yeorribirietiitvth the vcurrent voutpl'it -`of the 'power tube'toa mation characteristic which is line'ar'with-respect to time.

scanning is due 'to 'aidropping oif `of deilecticnlccil current and thevoltage'rnay be cilipeiisatedby controlling thev flowl of 'current'through gthe" d ifi-pver tube 3| "and thatas jare'sult of such controlrthen voltage fae'ress 'the beam deflection feoilsfaa 'rabola voltagewaveis utilized inthe automatic frequency controlI circuit. This voltageis' f'fsuch low impedance and magriltuue that "lt may "abe severelydifferentiated 'inthe ci "uitnvolvrg v'condenser "4l and resistor`43to"fo "asavtdth voltage as indicated. A suitable-sawtothvoltage isobtained evenwhn'resi'stcr 43 hasfaslw a value'as 330 ohms.

' his sawtooth voltage obtained fromthe differentiator yinvolvingcondenser 4 I and resistor M! is applied to a vclampingcircuit'finvolvingdiode li'and diode v441.

Although one formof clamping-circuit iseillustrated fanyfsuitable'arrangemen tfn'ray-be' employed to establish Aproper bias "level "iat 'Apoint g49 -vvhich lis dependent upon the 'relatlvephase'bi the-saw- Yin' detail inlthefpublishedfar It nesjoeen found that trie non-linearityfor nen published 'in the RCA Review for'March, 1948.

A clamping circuit of the type illustrated as applied to direct currentreinsertion in television systems is shown and described in my U. S.Patent No. 2,299,945, dated October 27, 1942.

The clamp circuit shown consists of the two diodes 45 and 41 driven bypush-pull pulses from the anode and cathode circuit of tube 9. The twodiodes 45 and 41 are driven through two condensers 5I and 53 and areconnected by two resistors 55 and 51 through appropriate filter circuitsinvolving resistors 59 and 6I and condensers 63 and 65. The timeconstant of the condenser 5I and resistor 55 circuit as Well as the timecon stant of condenser 53 and resistor 51 circuit is long compared tothe pulse time.

, The exact voltage to which condenser` 61 is charged or to which point49 is brought during the pulse time, depends upon the vvoltage acrossresistor 43 at pulse time. This will be understood when it isappreciated that the diodes 45 and 41 are biased such that theircathodes are positive with respect to their associated anodes during allthe time except during a synchronizing pulse when the cathode of diode45 is driven in a negative direction to the potential of the anode ofdiode 45 and the anode of diode 41 is driven positive to the potentialof its associated cathode.

It therefore follows that the potential at point 49 is dependent uponthe relative phase of the sawtooth voltage across resistor 43 and theincoming synchronizing pulses. A more detailed description of this clampcircuit action may be had through reference to United States patent2,358,545 to Karl R. Wendt, entitled Television System, issued September19, 1944. The frequency of the multivibrator involving tubes I1 and I9is thereby controlled in accordance with the relative phase of thegenerated sawtooth voltage with respect to the applied synchronizingpulses. If, for example, the speed of the oscillator involving tubes I1and I9 increases, the relative phase between the sawtooth voltage andthe synchronizing pulses will change to cause the oscillator to slowdown in frequency. l

Although the operation of this invention has been directed primarily tohorizontal deflection synchronization, it is not intended that theinvention be sufficiently limited thereto but may be employed in anyarrangement wherein accurate frequency control is necessary.

Having thus described the invention, what is claimed is:

1. In a television system of the type in which a deiiection power outputtube is driven by an oscillator which is frequency responsive to a biaspotential to deliver cyclically varying current to beam deflectionmeans, a frequency control for said oscillator comprising a source ofsynchronizing pulses, a source of anode potential for said power outputtube, means connected in the circuit between said power output tube andsaid source of anode potential for developing a recurring parabolicvoltage wave under the influence of the current drawn by said outputtube. means for differentiating said parabolic voltage to form recurringsawtooth voltage waves. means for comparing said recurring sawtoothvoltage Wave with said synchronizing pulses to develop a bias potentialdependent upon the relative phase of the sawtooth waves and thesynchronizing pulses, and a circuit connection between said lastmentioned means and said oscillator for applying said bias potential tosaid oscillator.

'2. In a. 'television receiving system of the 'type inwhich a deflectionpower output tube is driven by an oscillator which is frequencyresponsive to a bias potential to deliver cyclically varying current tobeam deflection means, a frequency control for said oscillatorcomprising in combination a synchronizing signal separator for saidsystem, a source of anode potential for said power output tube, aserially connected impedance` followed by a parallel connected capacityconnected in the circuit between said power output tube and said sourceof anode potential for developing a recurring parabolic voltage waveunder the influence of the current drawn by said output tube, meansfordifferentiating said parabolic voltage to form recurring sawtoothvoltage waves, means for comparing said recurring sawtooth voltage wavewith said synchronizing pulses to .develop a bias potential dependentupon the relative phase -of the sawtooth waves and the synchronizingpulses, and a`circuit connection between said last mentioned meansandsaid oscillator for applying said bias potential to said oscillator.

3. In a television system of the type in which a deflection power outputtube is driven by an oscillator which is frequency responsive to a biaspotential to deliver cyclically varying current to beam deflectionmeans,a frequency controlfor said oscillator comprising a synchronizing signalseparator, a terminal adapted to receive anode potential for said outputtube, means connected in the circuit between said power output tube andsaid terminal for developing a recurring parabolic voltage wave underthe influence of the current drawn by said output tube. an electricaldifferentiating circuit connected to said means for developing arecurring parabolicV voltage. means connected to said synchronizingsignal separator and said differentiating circuit for comparing saidrecurring sawtooth voltage wave with said synchronizing pulses todevelop a bias poten. tial dependent upon the relative phase of the,sawtooth waves and the synchronizing pulses, andfJ a circuit connectionbetween said last mentionedg, means and said oscillator for applyingsaid biasf;I potential to said oscillator.

4. In a television system of the type in whichvh a deflection poweroutput tube is driven by am oscillator which is frequency responsive toa bias,v potential to deliver cyclically varying current to., beamdeflection means, a frequency control for-vsaid oscillator comprising asynchronizing signali, separator for said system, a terminal adaptedto., receive anode potential for said outputtubeag, serially connectedinductance and a condenser ,forgdeveloping a recurring parabolic voltagewave under the influence of the current. drawn by said output tube,means for differentiating said parabolic voltage to form recurringsawtooth voltage waves, means for comparing said recurring sawtoothvoltage wave with said synchronizing pulses to develop a bias potentialdependent upon the relative phase of the sawtooth waves and thesynchronizing pulses, and a circuit connection between said lastmentioned means and said oscillator for applying said bias potential tosaid oscillator.

5. In a television system of the type in which a deflection power outputtube is driven by an oscillator which is frequency responsive to a biaspotential to deliver cyclically varying current to beam deflectionmeans, a frequency control for said oscillator comprising a source ofsynchronizing pulses. a clamping circuit for said synchronizing pulses,a source of anode potential for said wwercutput tube. an inductance anda condenser AI/Omficcted fin the circuit .between ,said power oui/- puttube and lsaid ysource of Aanode potential, an electricaldifferentiating circuit fconnected to @said inductance .and condenser, aconnection between said signal clam-ping -circuit and said source of`synchronizingvpulses, a vilter circuit connected t0 said clampingcircuit rand a bias lconnection Ybetween 'said oscillator andsaid ltercircuit. ,Y Y 6. vIn a television .system vin which a delectio .poweroutput :tube is adapted to :deliver vcyclically vvarying current'through :a coupling transformer Vto theA electromagnetic beam deection.means associated `with a cathode ray tube wherein 'an electron beam is.developed and then deflected by the .passage of said current throughYsaid delectionv means :so as tovscan a. `target area, and -invwhicl'i'rneans including a diode are provided in Ashunt with saidcoupling transformer for damping out oscillations which would normallybe produced during a portion'of yeachV current cycle in lpart by theinductance of vsaid coupling trans- -former and said deflection means,lan automatic frequency control comprising in combination a source'ofsynchronizing pulses, a source'of anode potential lfor said power outputtube, means con'- nected between said coupling transformer and saidsource of anode potential for developing a recurring parabolic voltagewave under the inviiuenceof the icurrentfdrawn by said output tube,means for differentiating said parabolic voltage to form :recurringsawtooth voltage Waves, lmeans -for comparing said :recurring sawtoothvoltage wave with said :synchronizing .pulses to rdevelop a biaspotential dependent upon the relative Iphase of the sawtooth Waves andthe synchronizing pulses, means for controlling the frequency ofoperation of sa-idpower output tube and la circuit connection betweensaid means for 4comparing said recurring -sawtooth voltage Wave withsaid synchronizing pulses and said means for controlling the 4frequencyof operation of said Vpower out- .put tube.

KARL R. WENDfl.

REFERENCES CITED The following references are of record in the l'e ofthis patent:

UNITED STATES PATE'TS Number Name Date 2,332,681 Wendt Oct. 26, 19432,344,810 Fredendall et al. Mar. 21, 1944 2,358,545 Wendt -Sept. 19,1944 2,450,155 Fredendall Jan. 4, 1949 OTHER REFERENCES Pcdgs. I. R. E.vol. 31, #1, January 1943, page 9.

